The demand for high-performance polypropylene homo- and copolymer products has resulted in advanced manufacturing technologies with unique features. One of the features is to have multiple sets of operating conditions during the polymerization. This allows multiple polymer components needed for different aspects of product performance, and/or the superior reactor operability and good product quality. Such multiple sets of operating conditions can be realized by the multi-stage polymerization (e.g., multiple reactors in series), multi-zone polymerization in a single reactor, or the combination of multi-stage and multi-zone polymerization process. In order to further differentiate the properties of the polymer components produced in such a process or for the purpose of process optimization, the operating conditions in different reactors or different reactor zones are often intentionally or unintentionally established under different flow regimes.
The term “flow regime” is a fluid mechanical concept which posses one or more unique features on the general flow pattern and/or system structure which distinguishes itself from other adjacent regimes. In single phase system, turbulent flow and laminar flow are the common examples of different flow regimes. In the multi-phase systems involved in advanced polymerization manufacturing, the situation is more complicated, and several flow regimes can be encountered, such as bubbling fluidization, turbulent fluidization, fast fluidization, pneumatic convey (dense-phase and dilute phase), packed moving bed, spouted bed and spout-fluid bed bed. Different flow regimes are differentiated by their flow patterns, phase distributions and holdups, heat and mass transfer, etc.
Different flow regimes maximize the opportunity in producing different polymer components. For example, the 2nd and 1st reactors used in The Dow Chemical Company's UNIPOL™ process to make impact copolymers are running under the turbulent fluidization regime and the lower end of the fast fluidization regime, respectively (WO2009029486). The Lyondellbasell's Spherizone reactor employs two reactor zones under fast fluidization and the packed moving-bed regimes, respectively (U.S. Pat. No. 5,698,642). Sabic Europe added a draft tube into the center of the UNIPOL™ reactor, to create a spout-fluid bed in which the annular zone is probably under turbulent fluidization regime and the zone within the center draft tube is likely under the dilute-phase fast fluidization regime (EP 1,196,238).
While the operation of multiple flow regimes within a reactor system offers benefits of product diversity, such systems may cause additional problems. In particular, a catalyst system which performs optimally under one flow regime may not perform the same when running under a different flow regime. For example, a catalyst system may operate properly in a high-velocity regime, but when in a low-velocity or high-solid-holdup regime could have operational problems such as particle agglomeration and formation of polymer “chunks”, believed to be mainly due to inadequately heat removal and/or static adhesion. Such operational problem is documented, for example, in EP1,720,913, WO2005/095465, U.S. Pat. No. 7,405,260. Therefore, there is a need to develop an improvement polymerization process to overcome the operational problems associated with the multi-regime reactor system.
Previous attempts to solve such operational problems include, for example, EP1,720,913, which describes the feed of multiple liquid streams continuously into a packed moving-bed zone of a polymerization reactor at certain mass flow rates for the “control of particle flow” and reactor pluggage prevention. Multiple liquid injection adds undesired complexity to the process and adds cost. In addition, many polymerization reactor systems involving multi-flow-regimes require the use of pre-polymerization. Pre-polymerization may improve dispersion of catalyst active sites and therefore reduce the probability of local overheating that may lead to the agglomeration of polymer particles; but it also adds additional investment and operational cost. Thus, there is a need to develop a solution to the operational problem in multi-regime polymerization reactor system that can be easily applied, and with relatively low cost and low operational complicity.
The present invention is an improvement for gas-phase polymerization processes employing two or more different flow regimes. The improvement involves the use of a mixed external electron donor feed.